The present invention relates to a catalyst for asymmetric epoxidation of enones and a process for producing an optically active epoxide employing it.
As a reaction for asymmetric epoxidation of enones, a method has been known wherein a compound having a carbonxe2x80x94carbon double bond conjugated to a carbonyl group, such as chalcone, is asymmetrically epoxidized with a hydroxyperoxide compound in the presence of a complex catalyst prepared from an optically active dihydroxy compound and a rare earth metal alkoxide.
Specifically, it is known to use a tetrahydrofuran solution of (R)-binaphthol and lanthanum triisopropoxide, as a (R)-lanthanum binaphthoxide, for the asymmetric epoxidation reaction (JP-A-10-120668).
However, by the method disclosed in JP-A-10-120668, if, for example, tert-butyl hydroperoxide (hereinafter referred to simply as TBHP) is used as an oxidizing agent, the desired optically active epoxide is obtainable only in a low yield and with a low optical purity. Therefore, in order to obtain a satisfactory result, it has been required to use a special binaphthol having methylol introduced at the 3-position, or to use an alkoxide of a rare earth metal other than lanthanum, such as ytterbium. Further, the amount of the catalyst is required to be from 5 to 10 mol % relative to the enone subjected to the reaction, and reduction of the amount of the catalyst has been desired.
The present applicants have already filed a patent application (JP-10-192743) for a catalyst composition comprising (A) an optically active binaphthol, (B) lanthanum triisopropoxide, (C) triphenylphosphine oxide, etc., which has a higher reactivity than the above-mentioned conventional catalyst and which presents a high optical purity to the product.
The present inventors have conducted a further study on a catalyst which presents a high optical activity and as a result, have found that a complex catalyst comprising (A) an optically active binaphthol, (B) lanthanum triisopropoxide, (C) triphenylphosphine oxide and (D) cumene hydroperoxide or tert-butyl hydroperoxide, is a catalyst which presents a higher optical activity to the product.
Further, the present inventors have conducted an extensive study to develop a catalyst which has a high reactivity and which presents a high optical purity and have found that a catalyst composition comprising (A) an optically active binaphthol, (B) lanthanum triisopropoxide and (c) tri(4-fluorophenyl)phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide, has a high reactivity and high stability as compared with the conventional catalyst and presents a high optical purity to the product, and further, it is thereby possible to reduce the amount of the catalyst relative to the enone subjected to the reaction. The present invention has been accomplished on the basis of these discoveries.
Namely, the present invention provides a complex catalyst for asymmetric epoxidation of enones, which comprises:
(A) an optically active binaphthol,
(B) lanthanum triisopropoxide,
(C) triphenylphosphine oxide, and
(D) cumene hydroperoxide or tert-butyl hydroperoxide;
a catalyst for asymmetric epoxidation of enones, which comprises:
(A) an optically active binaphthol,
(B) lanthanum triisopropoxide, and
(c) tri(4-fluorophenyl)phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide; and a process for producing an optically active epoxide of the following formula (1): 
wherein each of R1 and R2 which are independent of each other, is a C1-20 linear, branched or cyclic alkyl group, an aromatic group, an aromatic group substituted by from 1 to 5 C1-5 alkyl groups, an aromatic group substituted by from 1 to 5 C1-5 alkoxy groups, an aromatic group substituted by from 1 to 5 halogen atoms, a C1-5 linear, branched or cyclic alkyl group substituted by an aromatic group, or a C1-5 linear, branched or cyclic alkyl group substituted by a halogenated aromatic group, and symbol * represents optically active carbon, which comprises reacting an enone of the following formula (2): 
wherein R1 and R2 are as defined above, with an oxidizing agent in the presence of a such a catalyst.
Now, the present invention will be described in detail with reference to the preferred embodiments.
Firstly, the complex catalyst comprising (A) an optically active binaphthol, (B) lanthanum triisopropoxide, (C) triphenylphosphine oxide and (D) tert-butyl hydroperoxide or cumene hydroperoxide, will be described.
In the present invention, the constituting proportions of the above catalyst components are not particularly limited. However, (A) the binaphthol is usually from 1 to 3 mols, (C) the triphenylphosphine oxide is usually from 0.1 to 10 mols, preferably from 1 to 10 mols, and (D) the tert-butyl hydroperoxide or cumene hydroperoxide is usually from 1 to 20 mols, preferably from 1 to 10 mols, per mol of (B) the lanthanum triisopropoxide.
The above-mentioned components constituting the catalyst are added to a solvent which will be described hereinafter and maintained for from 0.5 to 4 hours within a range of from xe2x88x9250 to 100xc2x0 C., whereby a complex will be formed. By the formation of the complex, the solution will have a color of yellowish green to deep green.
In the present invention, it is preferred that a solution of the above catalyst complex is preliminarily prepared in the reaction system, and then the substrate to be subjected to the reaction and a necessary amount of an oxidizing agent are added to carry out the reaction.
In a case where the preparation of a solution of the catalyst complex of the present invention is not carried out, for example, in a case where (A) the optically active binaphthol, (B) the lanthanum triisopropoxide and (C) the triphenylphosphine oxide are mixed in a solvent without adding (D) the tert-butyl hydroperoxide or cumene hydroperoxide, and then the substrate to be subjected to the reaction and (D) the tert-butyl hydroperoxide or cumene peroxide are all together added to the system to carry out the reaction, it is likely that the yield will decrease, and the optical purity will decrease.
Now, the catalyst comprising (A) an optically active binaphthol, (B) lanthanum triisopropoxide and (c) tri(4-fluorophenyl)phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide, will be described.
In the present invention, the optically active binaphthol is specifically (R)-(+)-1,1xe2x80x2-bi-2-naphthol (hereinafter referred to as (R)-binaphthol) or (S)-(xe2x88x92)-1,1xe2x80x2-bi-2-naphthol (hereinafter referred to as (S)-binaphthol).
In the present invention, the constituting proportions of the above catalyst components are not particularly limited. However, (A) the binaphthol is usually from 1 to 3 mols, and (c) the tri(4-fluorophenyl) phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide, is usually from 0.1 to 10 mols, preferably from 1 to 10 mols, per mol of (B) the lanthanum triisopropoxide.
In the present invention, to the above catalyst, a predetermined amount of cumene peroxide (hereinafter sometimes referred to as CMHP) or tert-butyl hydroperoxide (hereinafter sometimes referred to as TBHP) may be added to form a complex showing a yellowish green to deep green color as a reactive species, and the mixture may be used for the reaction. However, in such a case, there is no substantial difference in the obtainable results. Further, to the above catalyst, CMHP or TBHP required for the reaction is preliminarily added, and then the enone may be added to carry out the reaction. However, also in this case, there is no substantial difference in the obtainable results.
In the present invention, the above-mentioned components constituting the catalyst may be added to a solvent which will be described hereinafter and maintained for from 0.5 to 4 hours within a range of from xe2x88x9250xc2x0 C. to 100xc2x0 C. to form a complex and then the substrate to be subjected to the reaction and an oxidizing agent may be added to carry out the reaction. Otherwise, a predetermined amount of an oxidizing agent is added to the formed complex, followed by stirring and then an additional oxidizing agent and the enone to be subjected to the reaction may be added to carry out the reaction.
The complex catalyst of the present invention is useful for a reaction for asymmetric epoxidation of enones, and it provides a high reactivity and presents a high optical purity to the product.
The optical absolute configuration to be developed by the reaction employing the complex catalyst of the present invention generally depends on the optical absolute configuration of the optically active binaphthol constituting the catalyst. Namely, with a substrate whereby the absolute configuration of the asymmetric carbon in the product becomes a (R) form when (R)-binaphthol is employed, the optical absolute configuration of the asymmetric carbon in the product will be a (S) form when (S)-binaphthol is employed. However, it is not always true that when (R)-binaphthol is employed, the optical absolute configuration of an asymmetric carbon in the product will be a (R) form, and the optical absolute configuration of the product varies depending upon the type of the substrate. In a case where an asymmetric epoxidation reaction is carried out by means of the catalyst of the present invention, when (R)-binaphthol is employed, the optical absolute configurations at the 2-position (xcex1-position of carbonyl group) and 3-position (xcex2-position of carbonyl group) of the epoxide of the enone, thereby formed, will usually be (2S,3R), while when (S)-binaphthol is employed, the optical absolute configuration will usually be (2R,3S).
In the process of the present invention, the amount of the complex catalyst is not particularly limited, but it is usually from 0.01 to 50 mol %, preferably from 0.1 to 25 mol %, based on the molar amount of the lanthanum isopropoxide, relative to the substrate to be subjected to the reaction.
As a solvent useful for the process of the present invention, any solvent may be used so long as it is a solvent inert to the catalyst and to the epoxidation reaction. However, an ether type solvent such as dimethyl ether, diisopropyl ether, 1,2-dimethoxyethane or tetrahydrofuran (hereinafter referred to simply as THF) is preferred from the viewpoint of the stability of the catalyst and the result of the epoxidation reaction, and among them, THF gives the best result.
The amount of the solvent is usually from 2 to 200 times, preferably from 5 to 100 times, by weight, the amount of the enone to be subjected to the reaction.
The enone useful for the process of the present invention may be any enone so long as it is a compound of the above formula (1). Specifically, methyl vinyl ketone, trans-3-penten-2-one, trans-3-hexen-2-one, trans-3-hepten-2-one, trans-3-octen-2-one, trans-3-nonen-2-one, ethyl vinyl ketone, trans-4-hexen-3-one, trans-4-hepten-3-one, trans-4-octen-3-one, trans-4-nonen-3-one, isopropyl vinyl ketone, trans-2-methyl-4-hexen-3-one, trans-2-methyl-4-hepten-3-one, trans-2-methyl-4-octen-3-one, trans-2-methyl-4-nonen-3-one, trans-1,3-diphenyl-2-propylen-1-one (chalcone), trans-2-methyl-5-phenyl-4-penten-3-one, 4-methyl-1-phenyl-3-penten-2-one, 4-phenyl-3-butylen-2-one, 6-phenyl-3-hexen-2-one or 5-phenyl-3-hexen-2-one, may be mentioned.
The oxidizing agent to be used in the present invention is usually CMHP or TBHP. However, other oxidizing agents may be used which have oxidizing powers of an equal level and which bring about no side reactions in the reaction system.
As TBHP to be used as an oxidizing agent in the process of the present invention, a commercially available solution in e.g. decane may be used as it is, or it may be extracted from a 70% or 90% aqueous solution with toluene, then dried over anhydrous magnesium sulfate and then used for the present invention. Likewise, as CMHP, a commercial 80 wt % product may be used after purification or as it is without purification. Further, depending upon the type of the substrate for the reaction, substantially quantitatively, a pure optically active substance can be obtained by using CMHP.
The amount of the oxidizing agent should theoretically be sufficient with an equivalent amount to the enone to be subjected to the reaction in the total of the amount used for the formation of the catalyst and the amount to be added during the reaction. However, to complete the reaction, it is preferably used in an amount of 1.1 times by mol.
In the process of the present invention, if the solution of the above-mentioned complex catalyst is used for asymmetric epoxidation of various types of enones, depending upon the type of the enone, there may be a case where the yield tends to be low, although a high optical purity will be given to the product. Accordingly, as a method for the reaction, a semi batch method is preferred to a batch method. Further, after adding the above enone to the preliminarily prepared solution of the complex catalyst, CMHP or TBHP is supplied to carry out the reaction, or to the preliminarily prepared catalyst solution, a mixture comprising the above enone and CMHP or TBHP, is supplied to carry out the reaction, whereby it is possible to obtain the desired product in a higher yield.
In the process of the present invention, it is important that the complex catalyst is present in a stabilized condition in the system. For this purpose, the ratio of the oxidizing agent (CMHP or TBHP) present in the system to the lanthanum element becomes important. If the supply rate of the oxidizing agent is slow, and the oxidizing agent in the system becomes deficient, the optical purity of the product tends to be low. On the other hand, if the supply rate of the oxidizing agent is high and an excessive oxidizing agent is present in the system, the yield may decrease. Accordingly, the supply rate of the oxidizing agent is adjusted to the consumption rate of the enone subjected to the reaction. If possible, it is preferred to determine the supply rate of the oxidizing agent after measuring the reaction rate of the enone subjected to the reaction in a reaction system similar to the practical operation.
In the process of the present invention, in a case where (C) the tri(4-fluorophenyl)phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide used as a component of a catalyst, the catalyst is more stable than a catalyst that is prepared with a case of triphenylphosphine oxide, so an amount of an excess oxidizing agent in the reaction system does not influence so much a stability of the catalyst as a stability of a catalyst that is prepared with the case of triphenylphosphine oxide. And in a case where (C) the tri(4-trifluorophenyl)phosphine oxide, tri(4-chlorophenyl)phosphine oxide or tri(4-trifluoromethylphenyl)phosphine oxide used as a component of a catalyst, an amount of the catalyst is able to reduce, relative to the substrate to be subjected to the reaction.
In the process of the present invention, in a case where an enone which is slow in the reaction, is subjected to the reaction, it is preferred to employ a method wherein the enone is added to a preliminarily prepared catalyst solution, and then the oxidizing agent is supplied thereto. On the other hand, in a case where an enone which is quick in the reaction such as chalcone, is subjected to the reaction, it is preferred to adopt a method wherein a mixture comprising the enone and the oxidizing agent is supplied to the preliminarily prepared catalyst solution to carry out the reaction.
In the process of the present invention, the reaction temperature varies depending upon the substrate of the enone, but it is usually within a range of from xe2x88x9250xc2x0 C. to 100xc2x0 C. As regards the reaction time, usually, the reaction will be completed within 24 hours.
In the process of the present invention, zeolite may be used as the case requires for the purpose of removing water in the system during the preparation of the catalyst or during the reaction, or for the purpose of accelerating the catalyst-forming reaction or the epoxidation reaction.
Such zeolite may be used in any ratio to the enone, but it is usually used in an amount of from 10 mg to 2 g per mmol of the enone. With respect to the type of such zeolite, various zeolites may be employed including type A zeolite represented by molecular sieve 3A, 4A or 5A, molecular sieve 13X, type Y and type L zeolites. Among them, molecular sieve 4A is preferred.
After completion of the reaction, post treatment or purification by e.g. column chromatography may be carried out to obtain the desired optically active epoxide in good yield with a high optical purity.
An asymmetric epoxidation reaction of an enone with high reactivity in good yield and with a high optical purity, is provided by the catalyst of the present invention. Accordingly, the process of the present invention is very useful as a process for producing an intermediate for various pharmaceuticals or agricultural chemicals.
Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples.
In the following Examples, the optical purities of the products were determined as follows.
Namely, the determination was carried out by using a high performance liquid chromatography having chiral column OB-H or AD of Daicel K.K. mounted, at a flow rate of 1 ml/min of an eluting solvent:
Hexane-i-PrOH=2/1-100/1 (vol/vol).
In the case of trans-2,3-epoxy-1,3-diphenylpropan-1-one, when using chiral column OB-H, an eluting solvent: Hexane/i-PrOH=2/1 is supplied at a flow rate of 1 ml/min, peaks of enantiomers of (2S,3R) and (2R,3S) appeared at retention times of 24 minutes and 32 minutes, respectively.